1. Field of the Invention
The invention relates to an optical article including a multilayered antireflecting coating wherein the last layer of the multilayered antireflecting coating has antifog properties.
The invention is based on the use of a layer-by-layer (LbL) coating to form the outermost layer of a multilayered antireflecting coating, said LbL coating comprising a superabsorbent polymer and being crosslinked to get a defined refractive index range.
2. Description of Related Art
It has always been difficult in the past to obtain coatings having at the same time good antifog properties and good antireflective properties.
U.S. Pat. Nos. 5,753,373, 5,873,931 and 5,997,621 disclose single antifog antireflection coatings made through sol-gel process of different surfactants, colloidal silica and crosslinking agent. However, such dip coated thin monolayer can hardly lead to a uniform antireflective (AR) function over an entire substrate and the antireflective performance is then limited to a quarter wave layer, with relatively high reflection level and low control of the residual color.
U.S. Pat. Nos. 6,958,172 and 7,153,584 disclose antifog antireflection coatings made through vacuum deposition of inorganic stacks with the last layer containing a mixture of inorganic silica, alumina and an organic compound having a hydrophilic group.
However, it is very difficult in practice to accurately control the deposition of the organic/inorganic hybrid layer in the desired ratios due to the concomitant evaporation of an organic material and an inorganic material. This process is difficult to use on an industrial scale.
Besides, the corresponding coatings become foggy upon rubbing.
In WO 2007/098843 and EP 1 987 378 there is disclosed a new concept for creating an antifog antireflection coating which comprises forming blind holes from the top of an antireflection layer to a water absorbing layer. The problem is that it is very difficult to make such small and uniform blind holes without impairing the optical and mechanical properties of the coating surface.
In WO 2007/056427, WO 2008/021817, US2008/0038458 there are disclosed superhydrophilic coatings formed from layer-by-layer (LbL) assembled films including nanoparticles and/or polyelectrolytes. These superhydrophilic coatings can be antifogging, antireflective or both antifogging and antireflective, especially by adjusting porosity of the coating.
The LbL coatings having antifogging and antireflective properties have generally a high porosity and a very low refractive index.
US2008/0038458 states that “a porous material can promote infiltration of water droplet into pores (to prevent fogging) and the pores can also reduce the refractive index of the coating so that it acts as an antireflecting coating”.
In the above cited patent application, a typical refractive index range is disclosed for a LbL coating obtained from positively charged TiO2 nanoparticles and negatively charged SiO2 nanoparticles layers alternately deposited. The refractive index of the LbL made of alternately deposited charged SiO2/TiO2 layers ranges typically from 1.28 to 1.32.
In US 2007014922, the disclosed range of refractive index for PAH (poly(allylaminehydrochloride))/SiO2 LbL (calcinated or not) is 1.24 to 1.32.
According to the above patent applications, an antireflective coating is obtained by depositing the LbL coating as a monolayer on a substrate.
In US patent 2009/0324910, there is disclosed a nanoporous coating including a first thickness having a first porosity and a second thickness having a second porosity.
The two porous layers of these coatings are made by a LbL process, using inorganic particles having different sizes. Differentially sized nanoparticles can pack so as to have different refractive indices.
Due to their high porosity, such antifog LbL coatings have poor mechanical properties.
Such highly porous antifog coatings show either poor adhesion, poor durability when formed on ophthalmic lenses made of organic glass or on hard coats classically formed on such ophthalmic lenses.
Further, it is usually difficult to design a durable antireflective stack having antifog properties through such a layer-by-layer process.
Sundareshan, in a thesis report entitled “influence of counter ions on antifogging coatings” having a “library” date Jun. 18, 2008, available on the MIT website, has studied the influence of counterions on ApSiO2/PAA LbL systems (ApSiO2 designates aminopropyl functionalized silica and PAA designates poly(acrylic acid)).
Indeed, such systems do not have yet sufficient long-lasting antifog properties.
The refractive index of a 114 to 129 nm thickness ApSiO2/PAA coatings has been modified by treating them with salt solutions of monovalent and divalent cations, namely sodium, lithium, potassium, calcium and barium salts.
The refractive indices of the ApSiO2/PAA coatings are decreased from 1.43 (unaltered LbL coating) to around 1.34-1.35 by the treatment with monovalent cations (Na+, Li+, K+) and were increased to 1.47-1.50 by the treatment with divalent cations (Ca2+, Ba2+).
Sundareshan suggested that the divalent cations may provide a source of internal crosslinking between NH2 and COOH groups, which can enhance the mechanical stability of the obtained ApSiO2/PAA LbL coatings.
Sundareshan noticed, however, that the treatment with such divalent cations, promoting crosslinking, decreases the hydrophilic properties and antifog properties compared to the initial non treated ApSiO2/PAA coatings.
There is still a need to provide novel antireflective coatings having antifog properties and having at the same time very good antireflective properties and mechanical properties versus the highly porous monolayered or bilayered AR LbL coatings of the prior art.
It is also desirable to provide a process for obtaining said antireflecting coating having antifog properties which involves only slight modifications of processes that are currently and widely used to make AR coatings in the field of ophthalmic lenses.